Cancer risk in relatives of BRCA1/2 pathogenic variant carriers in a large series of unselected patients with breast cancer

Objective: The spectrum and risk of cancer in relatives of BRCA1/2 pathogenic variant carriers in the Chinese population have not been established. Methods: A family history of cancer in 9903 unselected breast cancer patients was retrospectively analyzed. BRCA1/2 status was determined for all patients and relative risks (RRs) were calculated to evaluate cancer risk in relatives of the patients. Results: The incidences of breast cancer in female relatives of BRCA1 carriers, BRCA2 carriers, and non-carriers were 33.0%, 32.2%, and 7.7%, respectively. The corresponding incidences of ovarian cancer were 11.5%, 2.4%, and 0.5%, respectively. The incidences of pancreatic cancer in male relatives of BRCA1 carriers, BRCA2 carriers, and non-carriers were 1.4%, 2.7%, and 0.6%, respectively. The corresponding incidences of prostate cancer were 1.0%, 2.1%, and 0.4%, respectively. The risks of breast and ovarian cancers in female relatives of BRCA1 and BRCA2 carriers were significantly higher than female relatives of non-carriers (BRCA1: RR = 4.29, P < 0.001 and RR = 21.95, P < 0.001; BRCA2: RR = 4.19, P < 0.001 and RR = 4.65, P < 0.001, respectively). Additionally, higher risks of pancreatic and prostate cancers were noted in male relatives of BRCA2 carriers than non-carriers (RR = 4.34, P = 0.001 and RR = 4.86, P = 0.001, respectively). Conclusions: Female relatives of BRCA1 and BRCA2 carriers are at increased risk for breast and ovarian cancers, and male relatives of BRCA2 carriers are at increased risk for pancreatic and prostate cancers.

Most previous studies involving BRCA-associated cancer have focused on BRCA1/2 carriers; however, a limited number of studies have examined the cancer risk and spectrum in relatives of BRCA1/2 carriers. One study reported that relatives of BRCA1 and BRCA2 carriers have an increased risk for breast and ovarian cancers, and relatives of BRCA2 carriers also have an increased risk for pancreatic cancer 19 .
In addition, the data regarding the cancer risks in relatives of BRCA1/2 carriers are largely based on Western populations; scant data exists that pertains to Chinese populations.
Chinese individuals have significantly different cancer spectra and risks compared with Western individuals, with relatively lower risks of breast and prostate cancers and relatively higher risks of gastric, esophageal, and liver cancers 20,21 . Moreover, the spectrum of BRCA1/2 variants in Chinese women is different from Western women, with approximately 30% of variants not present in Caucasian women [22][23][24] . Therefore, the cancer risks in relatives of BRCA1/2 carriers in Chinese women might be different from Caucasian women. In the present study we aimed to clarify and characterize the spectra and risks of cancers in relatives of BRCA1/2 carriers in a large series of unselected Chinese patients with breast cancer.

Study population
Between October 2003 and May 2015, a total of 10,341 consecutive patients with breast cancer who were treated at the Breast Center of Peking University Cancer Hospital & Institute were enrolled in a breast cancer database. All patients were confirmed to have breast cancer based on preoperative biopsy or postoperative histologic examination. BRCA1/2 germline variants were determined in all patients by next-generation and/ or Sanger sequencing, or multiplex ligation-dependent probe amplification, as described previously [25][26][27] . All of the BRCA1/2 variants have been listed in detail in previous studies [25][26][27] . The BRCA1/2 status in 438 patients could not be determined due to insufficient quantity or poor quality of genomic DNA. Therefore, 9903 patients with breast cancer were ultimately enrolled in this study. Among these patients, four carried pathogenic variants in both the BRCA1 and BRCA2 genes, and were considered BRCA1 carriers at the time of analysis.
Variables, including demographics, clinicopathologic characteristics, and a family history of cancers, were extracted from the breast cancer database, and uncertain family history was confirmed and supplemented through telephone calls or outpatient visits.
Written informed consent was obtained during hospitalization from all patients whose clinical information and blood samples could be used for research, including genetic testing. The study was performed in accordance with the Declaration of Helsinki. Ethical approval for this study was obtained from the Ethics Committee of Peking University Cancer Hospital & Institute (Approval No. 2011KT12).

Definitions
BRCA1/2 variants are considered pathogenic when classified as pathogenic or likely pathogenic. Variants with uncertain significance (VUS) or likely benign or benign are considered non-pathogenic based on current American College of Medical Genetics and Genomics guidelines. Only patients with pathogenic variants were defined as carriers; other patients were considered to be non-carriers. A family history of cancers referred to cancers that occurred in first-, second-and third-degree relatives of the participants in our study. The cancer history of the participants and further relatives were not included in the analysis. A history of cancer in any relative that corresponded to the cancer in a study participant was regarded as a positive family history. Considering data accessibility, we used the family of the participants but not individuals as the unit with which to analyze the cancer risks of relatives; thus, cancer risks in relatives were calculated based on the family history of the study participants. Of note, a very small fraction of patients may have come from the same family, and such families may have been counted more than once. For convenience, related cancers with a low incidence were merged together for the analysis. For example, lymphoma included Hodgkin's lymphoma and non-Hodgkin's lymphoma, and urothelial carcinoma included bladder cancer and cancers of the ureter and urethra. Furthermore, fallopian tube cancer was considered ovarian cancer.

Statistical analysis
We analyzed all types of cancer with positive family history counts equal to or greater than five among BRCA1/2 carriers, and those with counts less than five were summarized as "other cancers". Continuous and categorical variables are described as the means ± standard deviations (SD) and frequencies with a percentage (%), respectively. Student's t-test was performed to analyze differences in continuous variables, the Wilcoxon rank-sum test was used to analyze differences in ordered categorical variables, and the Pearson chi square test or Fisher's exact test was used to analyze differences in unordered categorical variables. In addition, we calculated relative risks (RRs) with 95% confidence intervals (CIs) for comparison of cancer risks in relatives of BRCA1 carriers, BRCA2 carriers, and non-carriers. All statistical analyses were performed using StataSE 15 (StataCorp LP, College Station, TX, USA).
Only two-tailed P values < 0.05 were considered statistically significant.

Distributions of cancers in relatives of patients with breast cancer
There were 538 germline BRCA1/2 pathogenic variant carriers (209 for BRCA1 and 329 for BRCA2) and 9365 non-carriers in this study. The distribution of cancers in the relatives of the patients with breast cancer are presented in Table 1. The incidences of BRCA1 and BRCA2 carriers among relatives with 1, 2, and ≥3 cancer types were significantly higher than non-carriers ( Table 1). Additionally, the proportion of BRCA1 and BRCA2 carriers with 1, 2, and ≥3 affected family members was significantly greater than non-carriers ( Table 1). Differences in the number of cancer types and cancer-affected members among relatives between BRCA1 and BRCA2 carriers, however, were not significant ( Table 1).

Discussion
This is the first study to determine the cancer risks in relatives of BRCA1/2 carriers in the Chinese population. This study was based on a family history of cancer in 9903 unselected breast cancer patients. We found that the risks for breast and ovarian cancers in the female relatives of BRCA1 and BRCA2 carriers were significantly higher than non-carriers, and the risks for pancreatic and prostate cancers in male relatives of BRCA2 carriers were also significantly increased. In our study breast cancer was the most common cancer among female relatives of both BRCA1 and BRCA2 carriers. Ovarian cancer was the second and third most common cancer among female relatives of BRCA1 (21.95-fold increased risk) and BRCA2 carriers (4.65-fold increased risk), respectively. Additionally, our study demonstrated that relatives of BRCA1 carriers have a significantly higher risk for ovarian cancer (RR = 4.72) than relatives of BRCA2 carriers, suggesting that relatives of BRCA1 carriers should be more aware of ovarian cancer than BRCA2 carriers.
Pancreatic cancer is another common BRCA-associated cancer; however, multiple studies have shown that BRCA2, but not BRCA1, predisposes individuals to pancreatic cancer 3,7,11,19 . Our study findings were in agreement to these reports. Indeed, the pancreatic cancer risk was significantly increased in male relatives of BRCA2 carriers (RR = 4.34) but not relatives of BRCA1 carriers. Although pancreatic cancer risks were not shown to be significantly elevated in female relatives of BRCA1 or BRCA2 carriers, a trend toward risk for pancreatic cancer was still observed for female relatives of BRCA2 carriers (RR 2.54, 95% CI 1.02-6.30; P = 0.055). Notably, an increased risk for pancreatic cancer was reported in female BRCA2 carriers in a previous study 3 . Prostatic cancer is also associated with BRCA 3, [6][7][8][9][10][11]17,18,28,29 . Similarly, we found a significantly increased risk for prostate cancer in male relatives of BRCA2 carriers (RR = 4.86; P = 0.001), but a non-significant risk in male relatives of BRCA1 carriers (RR = 2.19; P = 0.241); our finding is in agreement with previous studies 3,7,9,11,17,18 .
In addition, we also found that the risks for liver and cervical cancers in female relatives of BRCA1 carriers, and lymphoma and kidney cancer in female relatives of BRCA2 carriers were significantly increased compared with female relatives of non-carriers. Moreover, the risks of esophageal cancer and lymphoma were markedly elevated in male relatives of BRCA1 carriers, and the risks of esophageal, kidney, and breast cancers in male relatives of BRCA2 carriers were also increased. It should be noted that the sample size of relatives with these cancers was small, and further studies are needed.
The cancer spectrum of relatives of non-carriers was similar to the common cancer spectrum of the general Chinese population 20,30 , which may accurately reflect cancer risks in the Chinese population. Several common malignancies in the general population, such as gastric, colorectal, and endometrial cancers, were not significantly associated with BRCA1 or BRCA2 variants in our study, even though correlations between these cancers and variants have been reported 4,6,7,11,13 . Our study findings did not rule out the possibility of increased risks for these cancers in relatives of BRCA1/2 carriers.
Several limitations should be considered when interpreting our study. Although the study involved a large cohort of unselected patients with breast cancer, the BRCA1/2 carrier sample size was not sufficiently large, and some types of cancer were rare after stratification by gender and gene type. Familial cancers were based on a family history of cancers; recall bias was inevitable, especially for the cancer histories among secondand third-degree relatives.

Conclusions
In summary, this study investigated the cancer spectrum and risk in relatives of BRCA1/2 carriers in a Chinese population. Our findings indicated that female relatives of BRCA1 and BRCA2 carriers are at increased risks for breast and ovarian cancers. The risk for ovarian cancer in female relatives of BRCA1 carriers is even higher than female relatives of BRCA2 carriers. In addition, male relatives of BRCA2 carriers have increased risks for pancreatic and prostate cancers. Our study may be useful for genetic counseling and cancer risk evaluation for relatives of BRCA1/2 carriers in the Chinese population.